The prior art discloses a wide variety of aromatic polyimide gas separation membranes which have high gas permeation rates. Among the prior art membranes are gas separation membranes made from a microporous and aromatic polyimide gas separation membrane in which the molecular structure is such that the molecules in the polymer can pack densely.
U.S. Pat. No. 4,717,394 discloses a broad class of aromatic polyimide gas separation membranes in which the monomeric diamines are rigid and substituted on essentially all of the positions ortho to the amino substituents and the monomeric acid anhydride groups are essentially all attached to rigid aromatic moieties. These polymers exhibit extremely high permeation rates to gases. U.S. Pat. No. 4,717,394 also teaches an aromatic polyimide gas separation membrane in which some of the before-described rigidity is relaxed through use of acid anhydride groups which are essentially attached to less rigid aromatic moieties, structurally less rigid aromatic diamines, and/or unsubstituted diamines. Through controlled reduction of the rigidity, polyimide gas separation membranes have improved selectivities to gases while still maintaining high permeation rates.
Such prior art membranes provide greater selectivity for the permeation of certain gases from multicomponent gas mixtures through the membranes. At comparable gas selectivities, membranes of the polyimides disclosed herein have generally high gas permeation rates than other polymers disclosed in the prior art. Such membranes do not, however, generally achieve high flux rates for condensable gases, particularly carbon dioxide, at low temperatures, while still maintaining good selectivity. The Polymer Handbook (Brandrup and Immergut editors) lists the activation energy of permeation for numerous polymers. The activation energy of permeation describes the temperature dependence of permeation. In particular, a positive activation energy indicates that the permeability increases with temperature. For all polymers listed, the activation energy of permeation is positive for carbon dioxide.
A dissertation entitled Gas Sorption and Permeation in a Series of Aromatic Polyimides (T. Kim, The University of Texas at Austin, 1988) reports the permeabilities of carbon dioxide and other gases for various polyimide membranes which are similar in structure to the invention membranes, at different temperatures. In particular, tables 15 and 17 of the dissertation report pure gas permeabilities of carbon dioxide and other gases for polyimide polymers made from (i) diaminofluorene (DAF) and hexafluoro isopropylidene bis (phthalic anhydride) ("6FDA") and (ii) isorpopylidenedianiline (IPDA) and 6FDA, as follows:
______________________________________ Temperature (.degree.C.) 35 45 55 ______________________________________ DAF-6FDA CO.sub.2 Permeability 1950 2020 2130 (centiBarrers) N.sub.2 Permeability 81.3 95.4 115 (centiBarrers) CO.sub.2 /N.sub.2 Selectivity 24.0 21.2 18.5 CH.sub.4 Permeability 35.5 46.0 57.0 (centiBarrers) CO.sub.2 /CH.sub.4 Selectivity 54.9 43.9 37.4 IPDA-6FDA CO.sub.2 Permeability 2430 2560 2740 (centiBarrers) N.sub.2 Permeability 87.2 109 139 (centiBarrers) CO.sub.2 /N.sub.2 Selectivity 27.9 23.5 19.7 CH.sub.4 Permeability 49.4 66.0 88.4 (centiBarrers) CO.sub.2 /CH.sub.4 Selectivity 49.2 38.8 31.0 ______________________________________
The permeability of carbon dioxide follows the well-established trend--productivity decreases with decreasing temperature.
Polymer membranes made from 2,4,6-trimethyl-3,3-phenylenediamine ("DAM") and 6FDA are known to show increased flux for carbon dioxide at low temperatures; however, in the vast majority of other cases, the polymers do not show such a temperature dependence and temperature must be carefully adjusted, usually increased, to achieve the desired flux rates and selectivities for each particular gas separation.
The present invention provides an aromatic polyimide membrane for the separation of condensable gases such as carbon dioxide, water, ammonia or hydrogen sulfide, particularly carbon dioxide, for other gases. The inventive membrane achieves high flux rates with good selectivity at low temperatures, a phenomenon heretofore unknown for this class of polyimide membranes.